This invention relates generally to phase shifted photomasks, and more particularly to a method for preventing undesirable null formation when the pattern on the phase shifted mask is transferred to the surface of an integrated circuit.
Phase shifting is a technique known in the art of photolithography to enhance the contrast of images printed onto the surface of an integrated circuit. Phase shifted photomasks are useful for allowing feature sizes of the printed images to be reduced below 0.5 .mu.m. A conventional and phase shifted photomask is shown in FIG. 1A. The conventional photomask includes a clear substrate 10 typically fabricated out of quartz, and a plurality of dark features 12 typically fabricated out of chrome. The dark features 12 represent metal runs, transistor contacts, transistor active areas, as well as all other patterned areas for a specific layer of the integrated circuit. The phase shifted photomask also includes substrate 10 and dark features 12, but in addition includes a number of additional clear phase shifting features 14 (also known simply as "phase shifters".) The phase shifters 14 are an extra patterned layer of transmissive material on the surface of the photomask. As light propagates through the substrate 10 and the extra phase shifters 14, its wavelength is reduced from that in air by the refractive indexes of the substrate 10 and the phase shifter 14, respectively. The optical phase difference, .theta., between two beams of light traveling through the phase shifted and non-phase shifted portions of the photomask is equal to: EQU .theta.=2.pi.a(n-1)/.lambda.,
wherein "n" is the refractive index and "a" is the thickness of the phase shifter 14. Usually, a phase shift of .pi. is desirable such that: EQU a=.lambda./(2(n-1)).
The phase shift can be any odd number of .pi., i.e. (2m+1).pi., where m=0,1,2, . . . . Phase shifting of the two beams is relative, but for simplicity of describing structure and operation, the raised area of the photomask is usually referred to as the phase shifter.
The phase shifted photomask in FIG. 1A is known as "alternating phase shifting" wherein pairs of closely packed dark features 12 contain a raised phase shifter 14. Separating each pair of dark features 12 is a non-phase shifted area of the photomask substrate 10. The operation of the alternating phase shifted photomask, contrasted with the operation of a conventional photomask, is shown in FIGS. 1B-1D. In sequence, the figures show the electric field on the mask, the electric field on the wafer, and the intensity on the wafer. The "-1" amplitude created by the phase shifters 14 effectively reduces the spatial frequency of the electric field so that it is less inhibited by the lens transfer function of the imaging system used and forms a higher-contrast amplitude image at the wafer plane. When the electric field is recorded by the photoresist, only the intensity that is proportional to the square of the electric field amplitude is recorded, resulting in doubling of the reduced spatial frequency, i.e. restoring to the spatial frequency of the original object but producing an image of much higher contrast. In addition to the reduction of spatial frequency, the electric field is forced to pass through zero to -1. Thus, edge contrast is improved. Therefore, the alternating phase shifting system benefits from the reduction of spatial frequencies as well as enhancement of the edge contrast.
Two types of alternating phase shifted photomasks are shown in FIGS. 2A-2B. FIG. 2A shows an "additive" phase shifted photomask in which the phase shifters 14 are a patterned layer of transmissive material as described above. FIG. 2B shows a "subtractive" phase shifted photomask in which the phase shifters 14A are formed by etching portions of the substrate 10. Thus, in the subtractive photomask, the unetched portions of the substrate 10 form the phase shifters 14. Although formed by different methods, the operation of the additive and subtractive photomasks is equivalent.
The alternating phase shifting technique works only as long as there are a number of pairs of closely packed dark features 12 on the photomask. As shown in FIG. 3, phase shifters 14 associated with pairs of dark features 12 must be eventually terminated. The phase shifters 14 are terminated because the dark features 12 are themselves terminated, change direction, are unconnected to other features on the photomask, or otherwise change topology. A phase shifter 14 is typically terminated in a transmissive end 16, resulting in a transmissive, optical clear edge. The optically clear edge 18 is shown in FIG. 4. The optically clear edge 18 results in a null, which produces a "stringer" or undesirable dark feature on the integrated circuit. Referring back to FIG. 3, the stringer is printed generally along the thin lines designating the end 16 of phase shifter 14. FIG. 4 only shows the termination of the phase shifter 14 due to the termination of the dark features 12. It is appreciated by those skilled in the art that the optically clear edge 18 and resulting stringers can occur in numerous other configurations.
The performance of a prior art phase shifted photomask is shown in FIGS. 5A-5C. The electric field produced by a conventional phase shifted photomask is shown in FIG. 5A. Although the edge 18 of the phase shifter is optically clear, the phase shifted light passing through the phase shifter passes through zero electric field. The light from the imaging system does not pass through the phase shifter exactly at the edge 18. The square of the electric field is shown in FIG. 5B, wherein the square generally has a value of 1, but is equal to zero at the location of edge 18. The intensity on the wafer is shown in FIG. 5C. The electric field on the photomask passes through a narrow zone of zero electric field, which is spread out due to the "point spread function" of the optical imaging system. However, the intensity is still relatively narrowly focused and is printed as an actual feature by the photoresist layer on the integrated circuit.
Accordingly, a need remains for a method of removing undesirable null formation in a phase shifted photomask while retaining the benefits of phase shifting.